1. Field of the Invention
The present invention relates to a novel high-technology method enabling precision creation and/or fitting of custom appendages to topographically complex two-dimensional and three-dimensional structures. The method comprises a combination of computerized tools and techniques employed to produce a virtually perfect fit with mating structures. The fitting method does not require the physical presence of the mating structure, and may be accomplished using only parametric dimensional and/or graphical data inputs that may be transmitted conventionally or electronically. If the mating structure is available, the method provides a means for direct digitization of the required topological data input. Novel elements of this method may also be used in conjunction with conventional fitting equipment and processes. The methods of this invention apply broadly to the general field of custom fitting appendages of all types. Two examples are presented to illustrate utility in the context of solutions to complex operations routinely encounter by gunsmiths, more particularly, in fitting recoil pads to gunstocks, and in fitting sighting ribs to gun barrels.
2. Related Art
There exist a countless number of applications that require elements or appendages to be custom fit to mating structures. Examples from range from a fierier fitting horseshoes, to the fitting of prosthetics limbs for handicapped people. Conventional methods typically require skilled technicians to manually form or machine the appendages to approximate a matching fit with the mating structure. Casting methods have been used extensively to capture and replicate the form-fit shape of mating structures and facilitate subsequent fitting by manual or mechanized duplication methods such as pantographs. In some cases the appendage can be fabricated directly from the cast material. While the number of fitting applications is too extensive to list, two examples have been selected to illustrate the utility of the methods of this invention in the fitting of appendages to complex three-dimensional structures. Moreover, these particular applications illustrate the advantages associated with fitting based on a combination of profile and parametric data inputs that are relatively easy to measure and record.
One example of the method of this invention involves an application wherein the process is used to facilitate fitting recoil pads or the like to guns. Recoil pads, butt plates, and spacers are commonly fitted to the butt stocks of long guns for the purpose of absorbing recoil, enhancing aesthetics, or to preserve or modify the fit of a gun by altering the geometry of the stock. Because recoil pads are typically thicker they accentuate the need for accuracy in fitting as a mismatched profile or angle will be readily apparent by creating an obvious surface discontinuity. The techniques used for fitting butt plates and spacers are essentially identical to the methods of fitting recoil pads.
There are a variety of reasons why recoil pads and butt plates should be replaced occasionally. The most common reason is due to the fact that the mechanical properties of most elastomers degrade with time and exposure to various environments and chemicals. Degradation of the elastomers can result in loss of elasticity, hardening, or embrittlement also known as dry rot. Many gun owners make the common mistake of storing their guns in a vertical orientation with the weight of the gun bearing on the recoil pads. Over time the pad may creep and deform into a distorted shape that no longer matches the contour of the stock and also reduces the ability of the pad to absorb energy.
In addition to the physical and chemical degradation of the pad, there are geometric situations that create the need to replace or refit a recoil pad. Any significant modification to the profile or position of the stock at the mating plane will disrupt the fit of the pad. For example, novice refinishers often remove the recoil pad in order to sand the stock in preparation for refinishing. In the process of sanding, the perimeter edges of the butt stock may be inadvertently rounded causing a mis-match of the recoil pad or butt plate to the stock. Subsequently, in an attempt to remedy this mistake, the stock may be cut or refaced in order to remove the rounded edges. Unfortunately, due to the taper of the gunstock the cutting operation will adversely change the fit. More commonly, stock alterations result from the intentional attempt to modify the dimensions or “fit” of the gun, or to preserve the original fit while adding a new recoil pad.
The unique and complex shape of the stock greatly complicates the fitting operations. The 3-dimensional shape of the typical butt stock can best be described as an asymmetrical frustum of a conic section. The perimeter edge of stock at the plane where the recoil pad is attached represents a complex two-dimensional profile with a unique ellipsoidal cross-section. This plane is typically flat as the result of a planar sawing operation. The multi-angular orientation and position of the butt plane defines a unique two-dimensional edge profile and three-dimensional adjacent surface contour.
Several recoil pad companies have marketed “pre-fit” recoil pads that are intended to fit the original contour of popular model guns. However, due presumably to loosely controlled manufacturing tolerances associated with both the gunstocks and the recoil pads, these pre-fit pads typically fit poorly. Moreover, if the original geometry of the stocks has been altered, pre-fit pads may not fit at all. For these reasons the majority of recoil pads are manufactured in the form of an oversize profile blanks intended for subsequent fitting of the pad to match the contour of the gunstock by sanding, grinding, abrading or other suitable material removal process hereafter referred to as machining.
The difficult task of conventionally fitting recoil pads to gunstocks is well known in the art and has necessitated the invention of many jigs, fixtures, methods, and specialized machinery. Most fixtures require the stock to be removed from the gun to facilitate handling dexterity and positioning of the stock. Removal of the butt stock itself may require the specialized tools and skills of a gunsmith. Conventional methods involve manually shaping the pad to a manually traced or scribed profile while the pad is held at the appropriate angles by a special jig or fixture. Topographical projections or discontinuities in the shape of the gunstocks such check pieces, and “stepped” or Monte-Carlo-type stocks, and protruding sling-swivel studs can interfere with the function of most conventional fitting methods. Pantograph type copy-milling or grinding devices have also been known to have be utilized for production and fitting of fit recoil pads. This process typically requires use of a “master” pattern such as a gunstock and/or recoil pad from which the new pad is reproduced.
In Pachmayr's U.S. Pat. No. 3,992,823, his mechanical apparatus facilitates the shaping of recoil pads to the stocks. Pachmayr's machine uses complex mechanical kinematics to control the relationship between the cutter and the recoil pad using the stock or a profile template a guide. Pachmayr claims that the ideal universal pivotal center of movement of his invention mechanism should be positioned midway between the convergence points of the top and bottom and two side surfaces of the stock. While Pachmayr's machine is acceptably effective, it is also complex in design, construction, set-up, and operation. Moreover, his machine comprises an inherent inaccuracy associated with the adaptation of a universal pivot point to approximate the complex motion required to precisely contour a recoil pad to a true form.
It is also known in the prior art that simplified jigs similar to Pachmayr's have been manufactured and distributed by the B-Square Company, and by B&R Inc. These jigs require scribing a line and subsequently facilitate manually grinding of the pad to the scribed line at one or more guided angles approximating the tapers of the stock. The angles are used as guides and the areas between are manually and therefore inaccurately blended.
While conventional jigs may reduce the risk of accidental damage, they do not eliminate the risk of nicking or scratching the gun during the pad fitting operation. In many cases the potential for accidental damage or loss of their guns or gunstocks in the process of fitting, transportation, or shipping, is sufficient to deter otherwise needy gun owners from replacing butt plates or recoil pads. This “risk of damage factor” is especially prevalent in the case of rare and valuable guns. Therefore, the method of this invention, which does not require removal of the stock from the gun, or use of the stock for fitting of the recoil pad, is a tremendous advantage.
A second example of the method of this invention involves an application wherein the process is used to facilitate fabrication and/or fitting of sighting ribs to the barrels of guns. In many shooting sports and hunting applications, the barrel is used as a sighting aid for the shooter who aims by viewing down the top of the barrel. Unfortunately, the surface of an irregularly contoured barrel does not provide a beneficial line of sight for aiming. The line of sight of provided by a tapered barrel is not parallel to the axis of the barrel and can give the perception of aiming-low which will result in the common effect of shooting high. In an effort to improve the “sight picture”, shotguns are frequently fitted with sighting ribs. The ribs provide a flat straight surface to aim down. The angle of the top plane of the rib with respect to the axis of the bore can be established in order to control the aim point.
The barrels of guns are typically tapered in a non-uniform contour. In order to minimize weight and maximize strength the wall thickness of a typical gun barrel is at a maximum around the chamber at the breech end, and decrease in thickness non-uniformly toward the muzzle. In the case of shotguns, the bore diameter also varies from chamber to muzzle most notably in the last two to six inches or “choke” which serves to focus the shot pattern. As a result of the complex geometry of the barrel, the operation of conventionally fitting and attaching a sighting rib to a barrel is very challenging.
Most ribs are permanently affixed to the gun barrel by means of solder, brazing, welding, or bonding. All of these methods of attachment require the precise machining of the rib to match the contour of the barrel in order to achieve an acceptable and consistent gap or bond-line. This can be a very difficult and time-consuming process using conventional measurement and manual machining methods.
Ventilated or vent rib have emerged as the most popular style rib. The lower section of a vent rib is intermittently scalloped out leaving a finite number of posts or piers for attachment to the barrel. The posts are typically very short, approximately one forth of an inch, in comparison to the scallops that are typically two to three inches long. Vent ribs are found on the majority of modern shotguns for several good reasons. The scallops or vents reduce weight and also help cool the gun barrel, which reduces the adverse optical effects of heat distortion. The reduced contact area also reduces the quantity and accuracy of the difficult machining required to fit the rib to the barrel.
Depending on the skill of the gunsmith, the resulting fit and finish of the ribs can vary greatly. Most gunsmiths neglect the effects of the barrel taper and machine each post to a straight average diameter at the center of the post. Alternately, they may machine a true cylindrical surface a slight taper angle relative to the axis of the barrel in an attempt to approximate the true irregular surface. Because of the extraordinary expertise and effort required, the cost of fitting a ribs is often not justifiable for the majority of guns which were not originally equipped with factory installed ribs. In the case of many mid-range and low cost guns, the cost associated with fitting a rib would often exceed the value of the gun itself. The efficiency and accuracy of the method of this invention will significantly reduce the cost of fitting ribs.